add migration info for Void -> EmptyResponse

[David-Development]

No description provided.

[David-Development]

@stefan-niedermann Here you said:

Calls are not affected by the EmptyResponse solution and can still use Void (or EmptyResponse) if wanted

I'm using Calls in the Nextcloud News App (e.g. here) and it received the error

Could not instantiate java.lang.Void, because response was null.

After changing it to EmptyResponse it worked. That's why I added a migration note for devs that run into this Problem.

[stefan-niedermann]

Hmm... I can't reproduce this issue 🧐 We are using Call<Void> successfully in the Deck app (at least on a branch).

Could not instantiate java.lang.Void, because response was null.

This error message suggests one is trying to instantiate Void (the workaround we removed in SSO 1.0.0) and it must not happen any longer. Are you absolutely sure you do not use SSO below 1.0.0 and also don't have idk any transitive dependencies or so?

[David-Development]

I cloned the Single-Sign-On Repo into the news app and included it as a gradle project dependency in the settings.gradle. So I'm fairly sure that it's version 1.0.0 🙈 And in the deck app it works with the latest version and the Call<Void>?

[stefan-niedermann]

On the observable2call branch, yup 🤷

Where exactly is the error message (Could not instantiate java.lang.Void, because response was null.) coming from? At some point Void must get instantiated - where is it?

[David-Development]

@stefan-niedermann Here: https://github.com/nextcloud/Android-SingleSignOn/pull/636/files#diff-c367b4028a9db1f826b9e7657605ad06e197238f3ab5e4a6d438dcd8c01d1983R151

The response is null (because the server doesn't return anything) so it checks if the the targetEntity is an Object - which it isn't so it throws the exception. Maybe in your case the server returns something and you just ignore it? That would probably be a different case

[stefan-niedermann]

One more thing: Please adjust the README, there's a hint to EmptyResponse which should be rewritten to also match Calls then

[stefan-niedermann]

Not sure whether the explanation is still valid (given Calls are not related to RxJava), but yeah, the hint is there.